In a film deposition device or a dry etching device in a semiconductor manufacturing process, special gas such as silane or phosphine, corrosive gas such as chlorinated gas, combustible gas such as hydrogen gas, or the like are used.
Flow rates of these gases should strictly be controlled.
The reason of this is because the gas flow rate directly affects a quality of the process. Specifically, the gas flow rate greatly affects a film quality in a film deposition process or a quality of a circuit processing in an etching process, whereby a yield of a semiconductor product is determined according to precision of the gas flow rate.
Another reason is that most of these gases are harmful to a human body and environment or have explosiveness. These gases are not allowed to be directly disposed in the atmosphere after they are used, so that a device used in a semiconductor manufacturing process should be provided with detoxifying device in accordance with a type of gas. However, the detoxifying device described above has limited processing capacity in general. Therefore, when the flow rate more than the allowable value flows, it cannot perfectly process the gas, so that the deleterious gas might be flown out in the atmosphere or the detoxifying device might be broken.
Moreover, since these gases, especially high-purity dust-free gas that can be used in a semiconductor manufacturing process, are expensive, and limitation is imposed on some gases for their use due to natural deterioration, they cannot be preserved in a large quantity.
In view of this, a known mass flow controller serving as a flow rate control device has conventionally been mounted in a semiconductor manufacturing process circuit so that a gas flows in an optimum flow rate for every type of gas. The mass flow controller described above changes the set flow rate by changing the applied voltage thus responding to changes in a process recipe.
However, these gases used in the semiconductor manufacturing process, especially the material gas for the film deposition among the so-called process gases, might cause precipitation of solid substances in a gas line due to its characteristics, so that the flow volume might be changed. The mass flow controller is formed with a capillary tube inside in order to supply a fixed flow rate with high precision. Even a small amount of precipitation of the solid substance on this portion could deteriorate the flow precision of the gas to be supplied. Further, since a gas with high corrosivity for an etching process or the like is flown, the corrosion of the mass flow controller cannot be avoided even if a material having a high corrosion resistance such as a stainless material or the like is used. As a result, a secular deterioration could occur, deteriorating the flow precision.
As described above, in the mass flow controller, the relationship between the applied voltage and the actual flow rate changes, so that the actual flow rate might possibly change. Therefore, the mass flow controller needs to be periodically subject to flow rate verification calibration.
The flow rate verification of the mass flow controller is basically performed by using a film flowmeter. However, this measurement is performed with a part of a pipe removed. After the measurement, the pipe should be assembled in the original state, and a leakage check should be executed. Therefore, the work is very time-consuming. Accordingly, it is ideal that the flow rate verification can be executed without removing the pipe.
As a method for performing the flow rate verification with the pipe assembled, there has been a method, as disclosed in Patent Document 1, in which a gas flow rate verification unit U is mounted downstream of a mass flow controller so as to constitute a gas mass flow measurement system. FIG. 19 shows a block diagram of a gas mass flow verification system.
As shown in FIG. 19, the gas mass flow verification system includes the gas flow rate verification unit U that includes a valve component 151, a chamber 153, a transducer assembly 154, and a valve component 152 and is mounted downstream of a mass flow controller 10. The chamber 153 has a known volume. The transducer assembly 154 is connected to a gas flow line 150 downstream of the chamber 153, and the valve components 151 and 152 are connected to the gas flow line 150 positioned downstream and upstream of the transducer assembly 154, thereby making the volume constant. The transducer assembly 154 outputs a signal directly indicating a PV/RT on the basis of pressure and temperature between the valve components 151 and 152. Here, P denotes a pressure, V denotes a volume, R denotes a gas constant, and T denotes an absolute temperature.
The gas mass flow verification system described above measures the actual flow rate of the mass flow controller 10 on the basis of the signal indicating the PV/RT outputted from the transducer assembly 154 without individually measuring pressure and temperature of the chamber 153. The gas mass flow verification system compares the actual flow rate with the preset flow rate of the mass flow controller 10, thereby verifying the flow rate of the mass flow controller 10.
Patent Document 1: Japan Patent No. 3022931